How a photo vault encrypts your photos, in plain English

"Encrypted" is one of those words that's on every privacy app and explained by almost none. Here's what happens, step by step, when you put a photo into VaultMaster, in words that don't need a maths degree.

Step 1: the vault gets a master key

When you create a vault, the app makes a long random secret, the master key. It's unique to that vault. Think of it as the one key that locks every item inside.

Step 2: your photo is scrambled with it

When you add a photo, video or file, VaultMaster scrambles it using AES-256-GCM and the master key. AES-256 is a very widely used standard. The GCM part adds a check, so if anyone alters the scrambled data, the app notices.

It isn't only the photo that's scrambled. File names, dates, thumbnails and the list of what you've stored are encrypted too, because those can reveal as much as the picture.

Step 3: the master key is locked away

If the master key sat on the phone in plain sight, the encryption would be pointless. So the key is stored only inside small files that are themselves locked. There are two ways to open them:

• Your pattern. You draw it, the app turns it into a key and uses that key to open the file.
• Your recovery phrase. A nine-word phrase opens a second small file holding the same master key, so the words can get you back in if you forget your pattern.

Step 4: turning a pattern into a key

A pattern is a weak secret on its own, because there aren't that many of them. So the app doesn't use it directly. It runs it through a function called PBKDF2 for 600,000 rounds, which deliberately takes work to calculate. That means someone trying to guess your pattern has to do that same work for every single guess, which slows them down a lot.

New patterns also need at least six dots, because longer patterns are harder to guess.

Step 5: a secret that never leaves the phone

The app also mixes in a separate secret, which is wrapped by a key held inside your iPhone's Secure Enclave. That's a protected chip-level area of the phone designed to keep keys safe. The result is that copying the vault's files, or even the phone's Keychain, doesn't give an attacker everything they need to guess your pattern on their own computer.

What about the recovery phrase?

The phrase has nine words and about 99 bits of randomness, which is far too many to guess, even with a copy of your files. It's the reason a vault can be recovered without a company in the middle, and also the reason you must keep it safe: if you lose it and your pattern, there is no way back.

What we never have

There's no account and no server holding your vault. We never receive your photos, your pattern, your recovery phrase or your keys. It's a deliberate design, and it means we couldn't help even if we wanted to.

What encryption doesn't do

Encryption protects your photos when the vault is locked or when someone has only the files. It doesn't stop someone watching you draw your pattern, taking a screenshot while the vault is open, or malware on an unlocked phone. We say so on the security page, because it's better you know.

Learn more

See the encryption page, the recovery phrase page and how to back up a vault and get back in. And if you're choosing between apps, here's what to look for in a private photo vault app.